Metal-organic frameworks (MOFs) are a remarkable class of relatively new materials that exist as a subclass of a larger group called coordination networks. MOFs have shown promise in a variety of applications ranging from gas storage to ion exchange. The materials are comprised of organic linkers that bridge discrete metal building units. Accurate structural knowledge is key to the understanding of the applicability of these materials, and thus the use of single-crystal x-ray diffraction is invaluable in this field of research. As research has identified potential industrial viability for these materials, there has risen a need to better understand their mechanical and physical properties. To study some of these mechanical properties, researchers used ALS Beamline 11.3.1 to perform in situ, high-pressure, single-crystal x-ray diffraction. This experimental setup allows researchers to study how compounds rearrange and react to variable hydrostatic pressure loadings, as well as to measure mechanical properties such as bulk modulus.

ZAG-4 viewed down the c-axis at ambient (left) and 7.32 GPa (right). Notice the likeness to a child safety gate with diamond-shaped channels.

Putting the Squeeze on MOFs

MOFs have revolutionized the field of crystal engineering and stand to potentially revolutionize the field of solid-state chemistry. The narrow definition of these materials is still under debate; however, the broad definition includes compounds exhibiting some level of dimensionality that are composed of metal-ions or cluster building units extended through coordinated organic linkers. Examples include carboxylates, phosphonates, and imidizoles. Similar to any other class of solid-state materials, these new compounds exhibit a wide variety of unique chemical properties attractive for a diversity of applications from ion- exchange to gas storage/separations.

Now that these compounds have presented themselves as being useful for engineering applications, more information is needed as to the mechanical properties and limitations. This information will lead to a variety of beneficial applications including pressure switches, smart body armor, pressure sensors, or shock absorbing materials. A recent review presented a number of cases where mechanical properties of hybrid materials had been investigated; however, that number was relatively small compared with the number of MOFs published to date. The main application front for MOFs has been around gas storage and the understanding of how these structures change with applied pressure will be important if these materials are to have any industrial viability.

The application of high pressure to a compound is important, as it provides information on phase changes or major structural shifts that could produce materials not currently obtainable through conventional means. It also allows us to understand how materials respond to external stress to provide information on their use in mechanical applications. If we can gain an understanding of how flexible frameworks react to high pressures, we can determine new applications for them that other, more rigid compounds may be incapable of surviving. One major benefit from this will be to study the structure property relationships and any anisotropy associated with these structure changes. A whole new family of piezofunctional materials may arise from the study of MOFs under pressure.

ZAG-4 viewed down the b-axis at ambient (left) and 7.32GPa (right). Notice how the alkyl linkers simply lay down without much distortion.

The sample chosen is an alkyl-chain supported MOF—Zn(HO3PC4H8PO3H)·2H2O. It has been named Zinc Alkyl Gate 4, or ZAG-4, for convenience. The compound was originally synthesized in 2003; however, no further work had been pursued. Researchers at Texas A&M stumbled upon the structure and noticed that when viewed down the c-axis it looked like a child safety gate. They had the idea to apply pressure to the compound in an attempt to open the gate.

By applying up to 9.9 GPa of external applied pressure in increments increasing from ambient pressure, researchers were able to study the structural deformation of ZAG-4. Single-crystal x-ray diffraction allowed for the solution of the crystalline structure at each pressure point as well as monitoring how the unit cell parameters were affected. Over the range of 7.32 GPa applied pressure, the unit cell volume decreases over 27 percent; however, after applying 9.9 GPa and then releasing the pressure and removing the sample, the unit cell parameters revert back to the original values from before the experiment, proving the structural resilience of ZAG-4. Included in the structural changes up to 7.32 GPa were some interesting characteristics: the b-axis of the system exhibits both positive and negative linear compressibility. Negative linear compressibility is the phenomena shown when a sample expands in one or more dimensions when external pressure is applied. What is truly interesting about ZAG-4 is that the compressibility of the b-axis changes, where the axis initially decreases then increases and then remains relatively stagnant. The structural changes associated with this correspond to a deformation and repositioning of the included water molecule in the channels that run along the c-axis. In addition to this, the inorganic chain-links begin to deform in a manner that causes them to compress in one direction and expand in the other.

The overall structural results from this experiment have shown that the alkyl-chains act as a spring-like cushion while allowing the inorganic portion to deform. This notion, given the correct design, may allow for researchers to develop new piezofunctional materials by incorporating metal ions that contain unpaired electrons to induce piezomagnetic moments among many other possible properties. This opens the door for MOF materials to find roles in a variety of applications such as pressure sensors, switches, and more.

Research funding: National Science Foundation, Texas A&M University Division of Graduate Education and Division of Materials Research. Operation of the ALS is supported by U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES).